Add article on debugging techniques for reql

Recently, there was a question on the mailing list asking about debugging a secondary index (to make sure it had the expected values in it). @danielmewes responded with this generally helpful technique:

secondary index debugging

An easy way to check which keys will be inserted into a secondary index is by running a .map(secondaryIndexFunction.default(null)) on the table. If you want to create a multi index, you can replace the map by concatMap.

Assuming that you want to create a multi index so that you can look up messages by the IDs of their responses (in JS):
r.table(...).indexCreate("resp_ids", function(x) { return x('response')('messages')('message-id'); }, {multi: true} )

other techniques

• turn a .filter into a .map to see what's being matched
• using r.expr to create dummy data without needing to create a table to test a query on

@neumino and @AtnNn both have a huge bag of reql tricks, some of which I bet are good for debugging

Start by locating the appropriate ReQL documentation section and review the supplied secondary-index example using map, concatMap, and indexCreate. Incorporate the listed filter-to-map and r.expr techniques, with the JavaScript examples, and consider the article complete when these debugging techniques are clearly documented.